Probing the Solubility of Imine-Based Covalent Adaptable Networks

Covalent adaptable networks (CANs) are polymer materials that are covalently cross-linked via dynamic covalent bonds. The cross-linked polymer network is generally expected to be insoluble, as is seen for traditional thermosets. However, in recent years, it has become apparent that—under certain conditions—both dissociative and associative CANs can be dissolved in a good solvent. For some applications (e.g., those that require long-term (chemical) stability), the solubility of CANs can be problematic. However, many forget that (selective) solubility of CANs can also be applied advantageously, for example, in recycling or modification of the materials. In this work, we provide results and insights related to the tunable solubility of imine-based CANs. We observed that selected CANs could be fully dissolved in a good solvent without observing dissociation of imines. Only in an acidic environment (partial) dissociation of imines was observed, which could be reverted to the associated state by addition of a base. By adjusting the network composition, we were able to either facilitate or hamper solubility as well as control the size of the dissolved particles. DLS showed that the size of dissolved polymer particles decreased at lower concentrations. Similarly, decreasing cross-linking density resulted in smaller particles. Last, we showed that we could use the solubility of the CANs as a means for chemical recycling and postpolymerization modification. The combination of our studies with existing literature provides a better understanding of the solubility of CANs and their applications as recyclable thermosets.


Overview of used chemicals
An overview of all used chemicals is provided in Table S1, including full names, abbreviations, purity and supplier, and were all used as received.
Table S1.Overview of all used chemicals, including full names, abbreviations, purity and supplier.

Synthesis of polyimine networks
All polyimine networks were prepared, according to our previously documented synthesis for polyimine CANs, S1 by dissolving terephthalaldehyde (TA, 4.00 mmol), tris(2aminoethyl)amine (TREN, 0.80 mmol) and either of the specified diamines (2.80 mmol) in a small amount of THF (typically ~5 mL per gram material).The solution was briefly mixed until a homogenous mixture was obtained.It was then poured into a glass petri dish, which was left for overnight.Most of the solvent evaporated to air, and a wet polymer film resulted.The film was further dried in a vacuum oven at 50 °C for at least one day to remove any remaining solvent and water.The formation of the polyimine networks was confirmed by FT-IR analysis when the C=O stretch signal of the aldehyde around 1670 cm −1 could no longer be observed and a new imine signal around 1640 cm −1 appeared.All polymer films had a typical yellow-to-orange appearance (see Figure S1), which is common to imine materials.

Synthesis of V-Urea networks
The synthesis of V-Urea networks was based on an earlier documented protocol from Du Prez and co-workers.S2 Ethylenediamine-N-N'-bis(acetamide) (EDABA, 4.00 mmol), TREN (0.80 mmol) and either of the specified diamines (3.20 mmol) were mixed together in 10 mL DMF.The mixture was carefully heated with a heat gun, while swirling the flask, to dissolve all material.Then, the mixture was poured into a petri dish, which was placed in an oven at 80 °C for at least 24 hours.The obtained polymers were further dried in a vacuum oven at 50 °C for overnight.The formation of the V-Urea networks was confirmed by FT-IR analysis when the C=O stretch signal of the acetamide around 1700 cm −1 could no longer be observed.The V-Urea materials had a yellow-to-orange appearance (see Figure S2).

NMR spectra of dissolved polyimines
The 1 H NMR spectra of the polyimine materials with different diamines are presented in the figures below.Assignment of the characteristic regions are mentioned in the captions under the corresponding figures.

Reversible dissociation of dissolved polyimines by addition of acid
The dissociation of imines is known to be catalyzed by acid, and as such we investigated if the polyimines would hydrolyze in more acidic environments.When decreasing the pH to stronger acidic environments we noticed that the imines started to partially dissociate to form a new equilibrium between imine and aldehyde and amine (Figure S11).Solutions were prepared of 0.1 M and 1.0 M acetic acid in 0.5 mL CDCl3, to which 5 mg of PI-30 was added. 1 H NMR analysis showed that an equilibrium between imines and hydrolyzed products formed instantly, where a higher concentration of acid resulted in the equilibrium being shifted more towards the dissociated products.These results show that in acidic conditions the imines do not fully dissociate, but instead the equilibrium between dissociation and formation of imines is shifted.We then also investigated if we could push the equilibrium back towards formation of the imines.For this, an equimolar amount of the base triethylamine was added, and indeed the equilibrium was rapidly pushed back towards formation of the imines, as only trace amounts (<1%) of aldehyde remained.

Scatter plots for the solubility of PI-30 as a function of solvent polarity and dielectric constant
The dissolved fractions for PI-30 in several common solvents was plotted as a function of the polarity (Figure S12) and dielectric constant (Figure S13) of the corresponding solvents in the figures below.

Imine bond exchange kinetic study
To study the kinetic of imine bond exchange reactions, first (E)-N-benzyl-1phenylmethanimine (BI), (E)-N-benzyl-1-(p-tolyl)methanimine (TI) and diamine (DI) were synthesized based on our previously reported paper.S4 For the transimination reaction between imine and amine, BI with initial concentration of [BI]0 = 14 mM and excess amount (imine:amine 1:30) of a commercially available diamine (DA) (structurally similar to the diamine used in the synthesis of polyimine networks) were dissolved in three different deuterated solvents (chloroform, DMSO, and acetonitrile) and followed by 1 H NMR spectroscopy at 25 °C, as shown in Figure S14.

Differential scanning calorimetry (DSC)
Differential scanning calorimetry was performed following ISO 11357 using a Perkin Elmer DSC 8000 which was cooled by a liquid nitrogen cooling system.Large volume (60 μL) stainless steel cups were used to hold the sample and used as a reference.The sample first equilibrated at -90 °C for 10 min and then heated to 150 °C with a heating rate of 10 °C/min, followed by a fast cooling (100 °C/min) to -90 °C.After equilibrating at -90 °C for 10 min, the second heating was performed again to 150 °C with the rate of 10 °C/min.
From the graph a Tg of 56 °C and melting point of 125 °C were observed for the sample Xyl30.These results also support out explanation regarding modulus increase in the rheological measurements and rule out that the modulus increase does not originate from crystallization.

Figure S1 .
Figure S1.Physical appearance of some polyimine materials, directly after the synthesis, without performance of any post-processing like hot-pressing.

Figure S2 .
Figure S2.Photo of V-Urea films directly after the synthesis, without performance of any post-processing like hot-pressing.

Figure
Figure S11. 1 H NMR spectra of PI-30 in CDCl3 (top), after addition of acetic acid (middle) and when neutralized with triethylamine (bottom).The spectra indicate that after addition of the acid the imines partially dissociate back to aldehyde, but after neutralization with triethylamine the equilibrium is again pushed fully towards imine formation.

Figure S12 .0Figure S13
Figure S12.Scatter plot of the dissolved fraction of PI-30 as a function of solvent polarity.The polarity values have been taken from literature.S3

Figure S14 .
Figure S14.Stacked 1 H NMR spectra of the transimination reaction with excess amount of DA as nucleophile in three different solvents at 25 °C, zoomed in the imine region (8.6 -7.5 ppm).The transimination reaction (conversion) is followed over time by integration of imine signals of the corresponding materials (shown asprotons "a" and "b" in the reaction scheme).Another peak that can be used to calculate the conversion is the ring protons of BI and newly formed benzylamine (shown as protons "c" and "d" in the scheme).The kinetic plots

Figure S15 .
Figure S15.Stacked 1 H NMR spectra of the imine metathesis reaction with excess amount of DI as nucleophile in three different solvents at 25 °C , zoomed on two different region, i.e. imine region (around 8.6 -8.1 ppm),and the proton next to imine (around 4.83 -4.67 ppm).The imine metathesis reaction (conversion) is followed over time by integration of proton signals next to imine of the corresponding materials (shown as protons "c" and "d" in the reaction scheme) due to better isolated peaks in this region.Another peak that can be used to calculate the conversion is imine protons of TI and newly formed BI (shown as protons "a" and "b" in the scheme), however, there are some overlaps in this region.The kinetic plots of the conversion over time in different solvents are presented in the bottom-right panel.Due to addition of the excess of nucleophilic reactant (DI), a pseudofirst order reaction could be assumed.The data was therefore fitted with the model for first-order kinetics to derive the rate constants listed inside the plot.

Figure S18 .Figure S19 .
Figure S18.Temperature sweep (left panels) and frequency sweep (right panels) measurements for the five different polyimine CANs.

Figure S20 .
Figure S20.DSC curve of sample Xyl30 showing the first (top) and second (bottom) heating run after a fast cooling step in the range of -90 to 150 °C.

Figure S21 .
Figure S21.Photos of pristine and recycled polymer films of PI-30.The black cross was drawn on the white paper below the petri dish to indicate the transparency of the polymer film.